1. Field of the Invention
The present invention relates to surface acoustic wave devices which utilizes a surface acoustic wave propagating through a surface of a piezoelectric substrate or a surface of a piezoelectric thin film provided on a predetermined base.
Recently, down-sizing of mobile communications devices such as portable telephone sets has been facilitated progressively, and there has been a demand for down-sizing of parts and improvements in the performance thereof. Normally, these communications devices are equipped with elements such as an oscillator, filter and a wave distributing element used to generate and branch a signal. Recently, there has been considerable activity in the research and development of devices using a surface acoustic wave in order to facilitate down-sizing of the above elements. Particularly, a surface acoustic wave filter in which an inductance is added to a surface acoustic wave resonator equipped with a reflector has a low insertion loss, and enables broadening of the band and high suppression performance. Hence, applications of such a surface acoustic wave filter to the next-generation automobile telephone sets and portable telephone sets have been considered.
2. Description of the Related Art
Surface acoustic wave (hereinafter simply referred to as SAW) devices use a surface acoustic wave that propagates through a surface of a dielectric body cut out of a piezoelectric crystal or a surface of a piezoelectric thin film. It is possible to control (adjust) characteristics of such SAW devices such as the central frequency and the pass-band range by changing the propagation speed of the SAW and/or the electromechanical coupling factor. The following first through third methods are known as means for controlling the characteristics of the SAW devices.
The first method is to select an appropriate piezoelectric crystal material, an appropriate cut surface of the crystal and/or the SAW propagating direction. For example, a crystal material such as LiNbO.sub.3 or LiTaO.sub.3 is used, and the X-112.degree. cut or 36.degree. Y-X cut surface of the LiTaO.sub.3 crystal is used.
The second method is shown in FIG. 1A. As shown in FIG. 1A, the second method is to provide an insulating film 3 between the surface of a piezoelectric substrate 1 (or the surface of a piezoelectric thin film) and a comb-type electrode 2 (see Japanese Laid-Open Patent Application No. 48-26452 or No. 52-16146). FIG. 1B shows a variation of the second method. In FIG. 1B, an insulating film 4 is formed on the surface of the piezoelectric substrate 1 (or the surface of the piezoelectric thin film) on which the comb-type electrode 2 is formed.
The third method is shown in FIG. 1C, in which an ion implantation layer 5 containing Ar, O.sub.2 or Si ions is formed in the surface of the piezoelectric substrate (or the surface of the piezoelectric thin film). The third method is disclosed in, for example, Japanese Laid-Open Patent Application No. 63-169806.
The first method changes not only the SAW propagation speed and the electromechanical coupling factor but also the SAW mode and the temperature coefficient of the SAW device. Hence, it is very difficult to optimize these parameters and thus produce devices having the desired characteristics.
The second method shown in FIG. 1A has a disadvantage found by an experiment conducted by the inventors. In the experiment, the insulating film 3 made of SiO.sub.2 (which is the most general insulating material) was formed between the surface of the piezoelectric substrate 1 and the comb-type electrode 2. FIG. 2 shows the result of the experiment, in which the horizontal axis denotes the thickness of the SiO.sub.2 insulating film 3 and the vertical axis denotes the electromechanical coupling factor k.sup.2 (%). The electromechanical coupling factor greatly depends on the thickness of the SiO.sub.2 film 3, particularly when the thickness of the SiO.sub.2 film 3 is equal or less than 1000 .ANG.. In other words, the value of the electromechanical coupling factor changes greatly due to even a small variation in the thickness of the SiO.sub.2 film 3, and barely changes when the thickness of the SiO.sub.2 film is greater than 1000 .ANG.. Hence, the setting of the thickness of the SiO.sub.2 film is very difficult. Further, absorption and damping of the SAW are caused by the SiO.sub.2 film, and hence there is a large insertion loss of a SAW device to which the second method is applied.
The variation of the second method shown in FIG. 1B has disadvantages similar to those of the second method.
The third method has a disadvantage in that the SAW propagation speed is changed by the thermal history resulting from a process carried out after the third method. Hence, the third method does not have good reliability, and the insertion loss of a SAW device to which the third method is applied is not negligible.